Given demand for synchronous electric motors, used in compressors, electric vehicles, hybrid vehicles, fuel-cell vehicles and the like, that are light and compact as well as high-output, low-vibration, low-noise, high-efficiency motors, a motor producing high torque with low torque rippling is particularly desired.
In a surface permanent magnet synchronous motor, where permanent magnets arranged on the surface of a rotor core, the torque produced by the permanent magnets (magnetic torque) is maximal when the magnetic fields produced by the permanent magnets have a 90° phase difference with the armature current, or in other words, when the current provided to the stator coils is maximum when the positional relationship between the inter-polar gaps on the rotor and the stator teeth around which the stator coils are wound is such that the two face one another. Any deviation from the 90° phase difference between the permanent magnet-produced magnetic fields and the armature current results in reduced torque.
Also, in an interior permanent magnet synchronous electric motor, where the permanent magnets are arranged inside the core, in addition to magnetic torque from the permanent magnets, reluctance torque is also produced due to the salient polarity owing to the difference in magnetic reluctance caused by the positions of the rotor and stator. Reluctance torque is maximal when the phase difference between the permanent magnet-produced magnetic fields and the armature current is in the neighborhood of 45°. Accordingly, the torque from an interior permanent magnet synchronous electric motor is a combination of magnetic torque and reluctance torque, and that torque is maximal when the phase difference between the magnetic fields and armature current is between 0° and the neighborhood of 45°.
Ordinarily, the torque of a synchronous electric motor features a ripple component that is based on the influence of the harmonic component of the permanent magnet-produced magnetic fields, the influence of the harmonic component of the armature current, and the like. As such, there exists technology for reducing torque rippling by mechanically offsetting the placement interval (angle) of the stator coils through which flows current in a single phase from the inter-polar interval (angle) of the rotor. Through the use of such technology, the phases of the torque pulsation produced by the stator coils are offset from each other and the torque rippling can be negated. As a result, a low-vibration, low-noise motor can be realized (examples cited in Patent Literature 1 and 2).
Patent Literature 1 discloses a synchronous electric motor in which the stator coils are concentrated coils, i.e. coils that are wound concentrically around a single stator tooth, the number of magnetic dipoles in the rotor is 10, and the stator teeth are arranged in two groups repeating a +U-phase, a −U-phase, a +V-phase, a −V-phase, a +W-phase, and a −W-phase, in that order, for a total of 12 teeth. In that example, stator teeth through which flows current of one phase (such as the +U-phase and −U-phase) are offset by an electrical angle of π/6 radians, so that the torque ripple produced by the respective stator coils is offset by π/6 radians. As a result, the torque ripple can be reduced.
Furthermore, Patent Literature 2 discloses the number of slots (equivalent to teeth) in which stator coils are arranged and where the number of magnetic dipoles on the rotor is such that a relation of 18 slots to 20 dipoles is satisfied. In comparison to a conventional synchronous electric motor, which has 12 slots to 8 dipoles or 9 slots to 8 dipoles, the cogging torque, or torque ripple that occurs when no current is flowing, can be reduced through such technology.
[Citation List]
[Patent Literature]
[Patent Literature 1]
Japanese Patent Application Publication No. H9-285088
[Patent Literature 2]
Japanese Patent Application Publication No. 2003-244915
[Patent Literature 3]
Japanese Patent Application Publication No. 2000-041392